SEOs are still debating whether keyword density is good for something. The most recent debate is at

Overall, the agreement is that is not useful.

Two issues that strikes me as these suggest a lack of understanding of how search engines work accomodate to the following questions:

1. Could KD be used by search engines or users to check for spam keyword?
2. Is Vector Space currently in use by modern search engines?

Let me clarify these points.

Could KD be used by search engines or web page creators to check for spam keyword?

Word repetition determined by search engines as spam keyword should be of more concern than what web page creators or a KD tool tags as spam keyword. After all search engines and not designers of web pages are the one that assign a rank to the documents. This goes with the user-machine relevance perception mismatch and the concept of document linearization as a gap analysis. We have thoroughly discussed both in our IRWatch Newsletter, at this blog, and at Mi Islita.

However, this does not mean end users are a zero to the left, as they are the ones that pay the bills. And even if they don’t, why rank high a page just to see users going to some place else after visiting it because is not suitable for human consumption? So, rather than using a KD tool, just write as natural and useful to your prospective clients and readers as you can.

Regarding the use of KD tools for checking for spam, this allegation reminds me of certain seo books, marketers, and community forums that insist in such non sense, just to keep their KD tools relevant and alive.

During the Web Mining Course we debunked almost on a rutinary basis these and similar SEO myths. For instance, grad students learned about several local weight models that attenuate frequencies, hence serving the purpose of both scoring local weights and dampening down the effect of keyword repetition. Two for the price of one!

This is more cost effective at neutralizing keyword repetition than computing and comparing against a whole new and extra ratio, KD. Best of all, it does not require of the two extra loops one would have to use to compute KD (one for every term i in a doc and another for every doc j across a collection). Thus, whatever the % ratio computed by a KD tool, it will be compacted/attenuated within the corresponding scales of the local weight model used. So, from the search engine side, KD is not even a cost-effective tool for fighting spam.

To be sure students understood, I included the following three questions in the Final Exam section that consisted of multiple choices. (The problem-solving section of the test is even more interesting, but is too long to include it here.)

#10. It is a false statement:

a. Distance is anti-similarity.
b. Keyword density estimates keyword relevance.
c. In Vector Space Theory, a document is a vector of terms.
d. In Vector Space Theory, a query is a vector of terms.

#15. Which model does not attenuate frequencies?


#16. Consider two documents d1 and d2 wherein local term weights are computed using the LOGA model. d1 repeats a term once. How many times this term should be repeated in d2 to triplicate its d1 weight? Assume Log 10 base.

a. 3 times.
b. 30 times
c. 100 times
d. 1000 times

Answers: 10. b, 15. b, and 16. c. (sorry I’ve made a typo).

Is Vector Space currently in use by modern search engines?

Suggesting the contrary is non sense. Vector Space models are used on a regular basis to score and rank documents. Implementation is not that hard across large collections if you use the right scoring system with updating and precaching techniques on a term-doc matrix. In fact, I’ll be teaching this Spring the graduate course Search Engines Architecture.

I will blog the syllabus tomorrow, but is already available from the Electrical & Computer Engineering and Computer Science Department of This is a lecture and lab session course. Students will build their own search engines, crawlers, parsers, stemmers, and vector space scoring systems using open source components and some of their own authorship.

On and on, SEOs still have no clue about what a search engine can or cannot do.